1. Technical Field
The present invention relates to a discharge lamp lighting device, a control method for the same, and a projector.
2. Related Art
As a light source of a projector, a discharge lamp such as a high-pressure mercury lamp or metal halide lamp is used. In such a discharge lamp, wear of electrodes due to discharge causes change in the shape of the electrodes. If plural protrusions are grown at the distal end of the electrodes or irregular wear of the electrode bodies progresses, shift of arc starting point and change in arc length occur. These phenomena cause reduction in luminance of the discharge lamp and shorten the life of the discharge lamp. This is not desirable.
As a method to address this problem, U.S. Pat. No. 6,815,907 proposes a technique of pulse width modulation with the absolute value of an AC current for discharge lamp driving maintained substantially at a constant level.
However, in U.S. Pat. No. 6,815,907, pulse width modulation of an AC current for discharge lamp driving is carried out in one modulation pattern. When the variation range of pulse width modulation is set to be large enough to sufficiently melt a protrusion grown at the distal end of an electrode, excessive heat load is applied to an electrode having no protrusion grown at its distal end, and this accelerates wear of the electrode. On the contrary, when pulse width modulation is carried out to such an extent that excessive heat load is not applied to the electrode having no protrusion grown at its distal end, if the protrusion at the distal end of the electrode becomes harder to melt because of increase in cumulative lighting time of the electrode and so on, the unwanted protrusion on the electrode cannot be melted, thus making it difficult to sufficiently restrain occurrence of flicker.